The present invention may be more easily understood in the context of low light imaging arrays such as those used in digital photography to record an image. For the purposes of this discussion, an image will be defined as a two-dimensional array of digital values that represent the amount of light received during an exposure period at each pixel on a two-dimensional surface onto which the image is projected. For the purposes of this discussion, it will be assumed that each pixel is a small rectangular area on that surface. In digital photography, the image is projected onto an imaging array in which each pixel includes a photodetector that measures the amount of light that falls on some portion of the pixel area.
In one class of imaging array, the detector utilizes an area of silicon to collect electrons that are generated by light that strikes the silicon. During the exposure period, the electrons accumulate in the pixel area. The charge collected in each pixel area is measured at the end of the exposure period by moving the charge to an amplifier and an analog-to-digital converter that provides a digital value for each pixel. The pixels are arranged as a plurality of columns of pixels. Each pixel in a column is part of an analog shift register. The image is readout by shifting the charge collected at each pixel through the shift register until it reaches the end of the column. The charge is then either input to an amplifier or moved to another shift register that finally deposits the charge at the amplifier. Imaging arrays of this type are often referred to as charge-coupled devices (CCDs). CCDs are characterized by large fill factors, since most of the area of each pixel is devoted to generating and storing electrons from the incident light, and hence, such devices have the potential for providing imaging arrays that can operate under low light conditions.
Image arrays that have high dynamic ranges are required for many applications, including conventional photography. The dynamic range of an imaging array will be defined to be the ratio of the maximum signal for a pixel to the minimum signal that is above the noise. Typically, the signal from each pixel is processed in a charge conversion circuit that converts the charge to a voltage that is, in turn, converted to a digital value by an analog-to-digital converter (ADC). In one embodiment, the charge-to-voltage conversion is performed by a capacitive transimpedance amplifier.
At low light levels, the quality of the image is set by the signal-to-noise ratio at each pixel. Hence, a conversion circuit having a low noise and a high gain must be utilized. If this conversion circuit is used to convert signals from pixels having high light intensities, the output voltages will be too high for conventional low cost CMOS circuitry. If, on the other hand, the amplifier gain is set to a low value to maintain the signal within the range of CMOS circuitry when the charge from high intensity pixels is processed, the noise levels of the amplifier will be too high to provide optimum performance for the pixels having small charges.
A high dynamic range imaging array also generates pixel values that require more bits to represent digitally. Both the cost of the ADC and the conversion time required to generate each pixel value increase with the number of bits. It should be noted that in a CCD array, each column of pixels is typically converted in a single conversion circuit by shifting the collected charge serially to the conversion circuit. Hence, the time to readout the image will be increased as the conversion time increases. While additional conversion circuits or high speed ADCs can be used to compensate for this increase in readout time, both of these solutions increase the cost of the imaging array.